1. Regulation and control of immune responses: Two differentiation factors were generated in syngeneic lymphocyte-macrophage cultures in which there was no requirement of antigenic or mitogenic stimulation for their production. These two factors were CCDF (cytotoxic cell differentiation factor) and TCDF (T cell differentiation factor). CCDF was required to induce the differentiation of IL-2 activated precursors into LICC (lymphokine induced cytotoxic cells). TCDF was required to induce the differentiation of antigen-or mitogen-activated CTL precursors into CTL effectors. In addition, TCDF was needed to maintain the cytotoxic activity of the fully generated CTL. 2. Tumor Immunology: LICC generated by lymphokines (IL-2 and CCDF) selectively killed lymphoid and solid tumor targets of different H-2 haplotypes and of different etiological origins. The LICC were also found to be very effective in preventing the in vivo growth of both lymphoid and solid tumors. The ability to generate LICC was preserved in the tumor bearing hosts until the terminal stage of tumor growth. The inability to generate LICC at this late state was not caused by deficient lymphokine production; rather the generation of suppressor T cells interferred with LICC induction. LICC appears to be one of the tumor bearing host's uncompromised defense mechansims against tumor.